The present invention relates to an optical package with an improved fiber alignment fixture and, more particularly, to a two-piece fiber ferrule which provides simplified axial alignment of the fiber to the active optical device and allows visible inspection of the stability of the fiber axial alignment fixation.
An important factor in the reliability of optical transmission systems is the stability of the various components--transmitters, repeaters, and receivers. The packages housing these components have been the subject of much study, especially the aspect of attaching and aligning the optical fiber to the active component (laser, LED, photodiode, etc.) contained in the package. The stability of these alignments over long periods of time is one of the most important components of the overall system reliability.
This is especially true for laser-based transmitters designed for high frequency, single mode transmission systems, where relatively small lateral movements between the laser and the fiber can result in large coupling losses. For example, a lateral motion of 2 .mu.m can result in a 1 dB coupling loss. Since many of these single mode transmitters are destined for use in remote locations (undersea, for example), the need to maintain precise and accurate alignment is critical.
An exemplary prior art optical package is disclosed in U.S. Pat. No. 4,119,363 issued to I. Camlibel et al on Oct. 10, 1978. In the Camlibel et al arrangement, the optical fiber is inserted in the package through an epoxy-filled tube. Upon solidifying, the epoxy fixes the position of the fiber relative to the tube. To achieve alignment between the fiber and the optical device, the tube is manipulated until maximum optical output is attained. The tube is then soldered into place by heating the entire package. An alternative package arrangement is disclosed in U.S. Pat. No. 4,623,220 issued to D. Grabbe et al on Nov. 18, 1986. Grabbe et al use a fiber alignment pedestal which is located inside the package next to the optical device. The fiber is fed through an epoxy-filled opening in the head of the pedestal and positioned axially to obtain maximum output. The epoxy is then cured to fix this axial alignment. The pedestal itself is located in an epoxy-filled well and is moved vertically and horizontally to maximize output. The epoxy is then cured in the well to maintain the pedestal in place. A problem with both of these arrangements is that the type of epoxy, solder, or other holding material used to fix the alignment must be carefully controlled; for example, materials with different melting temperatures must be used for the two separate alignments of Grabbe et al, so that the fiber and optical device will remain aligned during any further package processing. Additionally, extreme ambient temperature fluctuations during the operation of the device may cause the holding material to soften and thus disrupt the alignment.
As an alternative, U. K. Patent No. 2,124,402A issued to B. A. Eales et al on Feb. 15, 1984 uses laser welding in place of epoxy or solder, to fix the fiber in place. In particular, the optical fiber is encased in a metal tube and positioned on a heat sink next to the optical device. A metal clamp is placed over the fiber to hold it in position and is laser welded to both the heat sink and the metal cover around the fiber. Although the laser welds are more stable than the other forms of attachment previously discussed, the need to perform these laser welds inside the package and within close proximity of the active device may be troublesome.